JPH08211404A - Amorphous silicon pedestal liquid-crystal optical valve - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dispense with a CdTe light interrupting layer such as in a conventional liquid crystal optical bulb to reduce the manufacturing cost, and improve the spatial resolution and optical sensitivity. SOLUTION: This optical bulb has a liquid crystal layer 20, a plurality of photoconductive passages 14 formed out of amorphous silicon on the incident side of an input beam 8, an electric insulating material 16 with low dielectric constant for mutually insulating the photoconductive passages 14, and a terminal for supplying a potential 26 across the liquid crystal layer 20 and the photoconductive passages 14. The photoconductive passages 14 spatially modulate the potential crossing the liquid crystal layer according to the spatial characteristic of the input beam, and the insulating material 16 encloses the passages to form a potential barrier to the migration of charges between the passages. Further, each photoconductive passage 14 has a metal mirror pad 17 to prevent a read beam 10 from being incident on the photoconductive passage 14, and a dielectric mirror for reflecting mirror for reflecting the read beam 10 is provided on the mirror pad 17.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a liquid crystal light valve (LC
LV), especially a light valve with high photosensitivity and spatial resolution.

[0002]

A liquid crystal light valve (LCLV) is a light-to-light image transducer capable of receiving a low intensity light image and converting it in real time to light in an output image by light from another light source. LCLVs are used in a variety of military and commercial large screen graphic applications.

Reference (“Video-Rate Liquid Crystal Ligh
t-Valve Using an Amorphous Silicon Photodetecto
r ”, SID '90 digest, page No. 17A.2, 327-329
A conventional LCLV, such as the Hughes Liquid Crystal Light Valve described in P. (1990), provides a thick, continuous, uniform layer of amorphous silicon (a) as an optical substrate having a liquid crystal layer attached to the optical substrate. -Si) is used. A dielectric mirror is positioned between the optical substrate and the liquid crystal layer to reflect the read beam after propagating through the liquid crystal layer, and the CdTe
The blocking layer is located between the dielectric mirror and the optical substrate to prevent the read beam from energizing the optical substrate.

In operation, an AC voltage is supplied across the optical substrate and the liquid crystal layer. The input image beam is directed to the input side of the device and the read beam impinges on the read side of the device and is reflected. The input image beam energizes the photobase in the energized area, reducing the resistance of the photobase. This modulates the voltage across the corresponding area of the liquid crystal layer. Through this effect, the spatial intensity pattern of the input image is transformed into a spatial change in the voltage across the liquid crystal layer, which causes a spatial change in the orientation of the liquid crystal molecules. The orientation of the liquid crystal with respect to the read light deflection at a given position determines the amount of read light reflected from the light valve at that position. Therefore, when the read beam passes through the liquid crystal layer, it is spatially modulated according to the spatial modulation of the orientation direction of the liquid crystal.

[0005]

Ideally, a-Si
We expect to maximize the amount of voltage applied to the liquid crystal layer in response to changes in the resistance of the photoconductive layer. This state is achieved when the impedances of the liquid crystal layer and the a-Si layer (without light) match. Conventional LCLV achieves this by making the a-Si layer very thick (about 30 microns). Since the dielectric constant of a-Si is larger than that of the liquid crystal layer, a larger thickness is needed to match the impedance. When the a-Si layer is thickened, the distance between the input electrode and the read side of the device increases. This results in a smaller electric field across the liquid crystal and a-Si layer which reduces the response time, spatial resolution and spectral response of the device. Furthermore, a deposition time of about 30 hours is required to deposit a 30 micron a-Si layer, increasing manufacturing cost.

Another problem is that a thick a-Si layer induces stress on the glass substrate on which the LCLV is typically manufactured. This stress will bend the glass substrate, requiring a difficult and time consuming polishing process to achieve an optically flat surface. High stress levels also preclude the use of fiber optic substrates required in laser eye protection goggles and other display products.

LCLVs also suffer from "charge diffusion". In an ideal LCLV, the lateral resistance of the a-Si layer is high enough to prevent lateral migration of photogenerated charges. If the charge is allowed to diffuse laterally in the a-Si layer, the portion of the input image at one location in the a-Si layer will diffuse to adjacent locations, reducing spatial resolution. Conventional LCLV attempts to overcome this problem by increasing the resistivity of the a-Si layer by using a doped material. The p-type doping impurity such as boron is a
It is added to the a-Si layer so as to compensate for spontaneous generation of n-type doped impurities in -Si. A-
This method is very difficult because it must exactly compensate for the n-type doped impurities found to be effective in Si. Further complicating this is a
The amount of naturally occurring n-type doped impurities in -Si can change during the progress of deposition. This difficult reverse doping process increases the manufacturing cost and lowers the production rate.

Optical blocking CdT discovered in conventional LCLV
The e layer must be such that the read light leaking through the dielectric mirror does not enter the a-Si layer and overwhelm the spatially resolved input image. This CdTe layer has charge diffusion and intrinsic photoconductivity that contributes to the corresponding degradation of the spatial resolution of the output image. Furthermore, since CdTe is a toxic material, it requires additional processing and disposal costs, which makes LCLV
Increase the overall manufacturing price of.

In view of the aforementioned problems, it is an object of the present invention to significantly reduce the total number of photoconductive layer depositions, increase the electric field across the photosubstrate, remove the CdTe light blocking layer, and polish the substrate. The goal is to lower the manufacturing cost of LCLVs and improve their performance by simplifying and improving the production rate of the devices resulting from the low level of mechanical stress and the elimination of the difficult boron back-doping process.

Another object of the invention is the removal of the CdTe light blocking layer (LBL) used in conventional LCLV. The removal of LBLs further reduces manufacturing costs and improves spatial resolution and photosensitivity by removing one of the locations where charge diffusion occurs.

[0011]

These objectives are to replace a segmented and electrically isolated array of photoconductive pedestals in which a continuous photoconductive substrate (photosubstrate) is embedded in a low dielectric constant matrix material. Achieved by LCLV with new pedestal. The pedestal is constructed so that there is one pedestal per image pixel.
Since the photoconductive pedestal is surrounded by a heterogeneous matrix of low dielectric constant, the effective dielectric constant of the optical substrate is low. Due to the reduced effective dielectric constant, the impedance of the optical substrate can be matched to the impedance of the liquid crystal layer,
It has a photoconductive pedestal that is much thinner than conventional continuous light substrates.

[0012] The manufacturing cost of photobases is reduced as a result of the relatively short deposition times required to obtain thinner photoconductive films. In addition, thinner mechanically separated arrays of photoconductive pedestals exhibit less mechanical stress,
This allows the LCLV to be manufactured on fiber optic substrates, making it more productive and the complex polishing process used to correct curved glass substrates easier.

The thickness of the thin optical substrate used in the present invention is also conventional L due to the short distance between the input and counter electrodes.
It occurs in the optical substrate, which is an electric field 10 to 25 times higher than the CLV electric field. This improves response time, photodegradation rate, spatial resolution, spectral response of the LCLV, as high electric fields increase the rate at which photogenerated charges are swept out of the optical substrate bulk with respect to charge pair recombination rate. Is.

Moreover, because the photoconductive pedestal dielectrically separates the individual pixels, the charge diffusion problem observed in conventional LCLV is greatly reduced without the use of the difficult boron backdoping process. Or removed.

[0015]

These and other features and advantages of the present invention will be apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings. The present invention solves the aforementioned problems by providing an LCLV having a segmented and electrically isolated array of a-Si pedestals embedded in a low dielectric constant matrix material. The a-Si pedestal forms a photoconductive path between the input electrode and the liquid crystal layer, and the dielectric matrix forms a potential barrier to migration of charge between the pedestals. The dielectric matrix also lowers the effective dielectric constant of the photo substrate, which allows the photo substrate to be impedance matched to the liquid crystal layer using a thin layer of photoconductive material. The thin photoconductive layer creates a small gap between the input and the counter electrode. This creates a high electric field across the liquid crystal layer and the photoconductive layer, improving response time, photodegradation rate, spatial resolution and spectral response characteristics of the LCLV. Moreover, since the a-Si pedestal is electrically isolated from each other by the dielectric matrix, the charge diffusion problems observed in conventional LCLVs are greatly reduced or eliminated without the use of difficult back-doping processes. can do.

One embodiment of the present invention is shown in FIG.
An input image beam 8 is directed to the input side of the device and a read beam 10 is incident on and reflected from the read side of the device. First transparent electrode 12 such as indium tin oxide (ITO)
Are deposited on the glass substrate 13. An array of a-Si photoconductive pedestals 14 is made of a heterogeneous low dielectric constant matrix material 16.
And extends from the first electrode 12 toward the read side of the device. The pedestal 14 is preferably 4-5 microns thick. The dielectric matrix material 16 is preferably SiO ₂ , which can be ion sputtered but other low dielectric constant materials such as polyimide, Si
N _x, BN, may be BeO or diamond-like carbon. The main requirement for the dielectric matrix material is a-Si
Having a low dielectric constant with respect to (having a dielectric constant of 12.7 to 13.0) and of the pedestal 14 isolated by direct photolithography or by the resulting replication using a separate photoresist and etching step. It is configurable into an array. The dielectric matrix material 16 does not corrode the a-Si pedestal 14 and does not contaminate the a-Si in the pedestal region,
It must be possible for the stack to withstand the rather harsh chemical and thermal environments associated with a-Si plasma deposition processes without separation during the removal process.

The a-Si pedestal 14 is physically and electrically isolated from each other by a dielectric matrix material 16.
It is protected from the read beam 10 by a metal mirror pad 17. The mirror pad 17 forms a metal matrix mirror that helps reflect the read beam 10. Each pedestal / mirror pad combination constitutes one image pixel. Therefore, the cross-sectional area of the pedestal 14 and the mirror pad 17 can change depending on the desired resolution. Furthermore, a-Si
Other factors such as the dielectric constant of the insulating matrix 16 of the film and dark resistance can affect the space of the a-Si pedestal 14. Usually required to achieve the effective dielectric constant of the dielectric matrix / a-Si pedestal combination and the specific switching ratio (ratio of current due to light present in a-Si to current present without light). It is preferable to minimize the cross-sectional area of the pedestal 14 to reduce the thickness of the a-Si pedestal 14.

A plan view of the pedestal / mirror pad combination is shown in FIG. A-Si in a typical device
The pedestal 14 is square and 4 microns on a side, the mirror pad 17 is square and 18 microns on a side, and the center-to-center spacing between the pedestals 14 is 20 microns.
The pad 17 rises to the potential of each pedestal 14 and sets an image pixel.

Referring again to FIG. 1, dielectric mirror 18 is fabricated on top of metal pad 17 and dielectric matrix 16.
Dielectric mirror 18 is preferably about 2 microns thick.
This reflects the read beam 10 and prevents it from entering the dielectric matrix 16 through the open areas between the metal mirror pads 17. Such stray light would otherwise be generated by the dielectric matrix 16 in the a-Si pedestal.
Scattered to the side of 14, which causes an accidental bias of the pedestal 14. The dielectric mirror 18 and mirror pad 17 replace the CdTe light blocking layer used in conventional LCLV's, which is difficult to process and degrades the performance of the LCLV '.

The liquid crystal layer 20 exists between the dielectric mirror 18 and the second ITO electrode 22. The glass plate 24 is the second electrode
Installed on 22.

In operation, the AC voltage source 26 is an ITO electrode.
A voltage is applied across the liquid crystal layer / a-Si layer combination through 12,22. When the writing light is not incident on the input side of the a-Si pedestal 14, the voltage drop is mainly due to the high resistivity of the a-Si pedestal 14 and the matrix material 16, which is mainly the liquid crystal layer 20 and the a-Si pedestal 14 and the matrix material. It occurs across 16. When the area on the input side of the a-Si pedestal is energized by the input light, electron-hole pairs are generated and the electric field of the a-Si pedestal 14 causes
Swept in the direction of the input and read end of the Si pedestal 14. The generation of electron-hole pairs is aS at the bias position.
The specific resistance of the i pedestal 14 is reduced. As a result, A
The C voltage is supplied to the corresponding position of the liquid crystal layer 22.

By this effect, the spatial intensity pattern of the input image is transformed into a spatial variation of the voltage across the liquid crystal layer 22, which produces a spatial variation in the orientation direction of the liquid crystal molecules. The orientation of the liquid crystal with respect to the reading light polarization at a given position determines the amount of reading light reflected from the light valve at that position. Therefore, when the read beam 10 passes through the liquid crystal layer, it is spatially modulated in response to the spatial modulation of the orientation direction of the liquid crystal.

FIG. 3 shows another more preferred embodiment, in which the microlens array 28 is attached to the glass substrate 13 with optical cement (not shown). Lens array 28 includes one microlens 30 for each a-Si pedestal 14. Lens 30 improves the photosensitivity of the device by collecting a portion of image beam 8 that would otherwise fall outside the pedestal 14 area and directing this light directly to a-Si pedestal 14. In the ideal case of a perfect photoconductor, the lens array 28 would be the area of the lens 30 and the pedestal.
It increases the photosensitivity of the device by the ratio of the area of 14 regions. The microlens array is preferably constructed using refractive optics, but may be constructed with a binary grating.

A preferred method of manufacturing the pedestal is shown in FIG.
Through i. First, as shown in FIG. 4a, a layer of indium tin oxide 12 was deposited using standard ion beam sputtering techniques on a glass substrate 13, preferably 3.5 inches thick. It The ITO layer 12 is preferably deposited to a thickness of 0.04 micron.

In the next step (FIG. 4b), the dielectric matrix material 16 (preferably SiO ₂ ) is ion beam sputtered onto the ITO layer. The thickness of this layer is finally a
Equal to the thickness of the Si pedestal, preferably 4 to 5
Micron.

The next step (c in FIG. 4) is to deposit a layer of photoresist 32 onto the dielectric matrix material 16, preferably to a thickness of 1-2 microns, which is exposed by standard photolithographic techniques, The step of rinsing is left to leave separate holes 34 in the photoresist 32 that set the position of the resulting pedestal.

The dielectric matrix material 16 is etched through the holes 34 in the photoresist 32 by plasma etching or wet chemical etching techniques to remove the matrix material.
Form a hole 36 similar to 16. The remaining portion of the matrix material is protected from the etchant by the photoresist 32, which is removed after the formation of the holes 36 in the matrix material 16. Etching of holes 36 exposes the underlying portion of ITO layer 12. The resulting structure is shown in Figure 4d.

In the next two steps (e-f in FIG. 4), the a-Si layer 38 is deposited using the well known plasma enhanced chemical vapor deposition technique. Sufficient a-Si is deposited to fill the holes 36 in the matrix material 16. Excess a-Si 38 is mechanically polished away, leaving the a-Si pedestal 14. Matrix material 16 and pedestal 14
Are polished to obtain an optical plane.

A second layer of photoresist 40 is then deposited, patterned, and rinsed away, leaving pad openings 42 in photoresist 40, as shown in FIG. 4g.

In FIG. 4h, a metal, preferably aluminum, having a thickness of 0.1 to 0.4 is deposited in the openings 42 of the photoresist mask. The photoresist mask is washed away, leaving a separate metal-mirror pad 17 on top of each a-Si pedestal 14. Finally, a dielectric mirror 18 is deposited, preferably 2 microns thick, as shown at i in FIG.

While some exemplary embodiments of the present invention have been shown and described, various modifications and alternative embodiments will occur to those skilled in the art. Photoconductive pedestals, for example, are made from other types of photoconductive materials. Such modifications and other embodiments can be made without departing from the scope of the present invention.

[Brief description of drawings]

FIG. 1 a-Si pedestal L constructed according to the present invention
Sectional drawing of CLV.

FIG. 2 is a sectional view taken along the line 2-2 in FIG.

FIG. 3 is a cross-sectional view showing an a-Si pedestal LCLV embodiment incorporating a lens array.

FIG. 4 IT for a-Si pedestal LCLV structure
Sectional drawing which showed the continuous step of manufacture of O input electrode, a-Si pedestal, a metal matrix mirror, and a dielectric mirror.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Chun-Shen Wu Ashwood Avenue 3536, Los Angeles, CA 90066, USA 3536

Claims (9)

[Claims] 1. A liquid crystal light valve having an input side for receiving an input light beam and a read side for receiving a read light beam, comprising: a liquid crystal layer; a plurality of photoconductive passages on the input side of the liquid crystal layer; An electrically insulating material between the passages to insulate the passages from each other, and a terminal for supplying a potential across the liquid crystal layer and the photoconductive passages, the photoconductive passages being spaces of the input beam. Responsive to an input light beam to spatially modulate the potential across the liquid crystal layer in response to an optical characteristic, the electrically insulating material surrounds the passages to form a potential barrier to migration of charges between the passages. Liquid crystal light valve that is characterized by. 2. The liquid crystal light valve according to claim 1, wherein the terminal includes a transparent input electrode on an input side of the photoconductive path and a transparent counter electrode on a read side of the liquid crystal layer. 3. A liquid crystal light valve according to claim 2, wherein the electrically insulating material comprises a layer of dielectric material on the read side of the input electrode. 4. The liquid crystal light valve of claim 3, wherein the photoconductive passage comprises a plurality of photoconductive pedestals extending through the dielectric material. 5. A conductive and optically reflective pad extending over the pedestal between the read side of the pedestal and the liquid crystal layer, the pad further comprising an underlying photoconductive pad. 5. A liquid crystal light valve according to claim 4, forming an array of reflective pixels that reflects the read beam from a pedestal. 6. A dielectric mirror is provided between the pad and the liquid crystal layer and between the dielectric material and the liquid crystal layer,
The liquid crystal light valve of claim 5, wherein the dielectric mirror further reflects the read beam from an underlying photoconductive pedestal. 7. The liquid crystal light valve according to claim 1, further comprising an image forming device that separates the input beam into a plurality of image pixels and focuses the image pixels on respective photoconductive paths. 8. The imaging device comprises a transparent material layer on the input side of the photoconductive path and a lens array on the input side of the transparent material, each lens of the lens array being the input beam. 8. The liquid crystal light valve according to claim 7, wherein the focal point of each part of the above is connected to each photoconductive path. 9. The liquid crystal light valve according to claim 4, wherein the photoconductive pedestal is formed of amorphous silicon.